The present invention relates generally to three-level converters and, more particularly, to a voltage clamp snubber for use in three-level converters.
Snubbers have been designed to provide safe switching of GTO (gate turn-off thyristor) power switches in both two and three level converter (inverter or rectifier) topologies. A GTO switch requires local control of dv/dt (voltage changes with respect to time) during switch turn-off which typically is accomplished by forming a shunt capacitor path around the switch. The resulting snubber arrangements for three-level converters are complicated and, in addition to creating parasitic energy losses, generally result in significant power losses as the local shunt capacitors dump their charge into the GTO during switch turn-on. Attempts have been made to design regenerative snubbers circuits to recover this energy. In general, these regenerative schemes are considered complex.
Voltage clamp type snubbers have been successfully used in two-level BJT (bipolar junction transistor) and IGBT (insulated gate bipolar transistor) converters. These switches do not require dv/dt control. Consequently, the clamp snubber can be used to limit switch overrings and thereby create a more robust converter.
IGCT (Integrated Gate Commutated Thyristor) switches can be operated in a converter without snubbers or clamping circuits. However, in a practical high power converter circuit, the parasitic inductance levels necessary for true snubberless converter operation cannot easily be achieved.
A neutral point clamped (NPC) converter can be operated with conventional resistor-capacitor-diode snubbers around each switch as shown in FIG. 2 of Kawakami et al., "A Large Capacitor GTO Inverter with Low-Loss Snubber Circuits," IEEE May 1991 publication number 0-7803-0453, pages 892-897. There are several limitations with this embodiment including the fact that the snubber capacitor completely discharges at each switch turn-on, involving a significant CV.sup.2 /2 energy loss each switching cycle with no practical benefit. Another limitation is that the asymmetric nature of the clamp diodes in the three-level inverter topology introduces a voltage imbalance between the inner and outer switch snubber capacitors and thereby decreases the switch safety margin.